The present invention relates to a device for judging the strength of knocking produced in an internal combustion engine.
It is known that a continuous occurrence of strong knocking in an engine will affect the durability of the engine; however, light knocking at relatively low engine rotational speeds results in optimum conditions for the engine output and fuel cost characteristics. The ignition timing has a very important relationship for the production of knocking and in general the tendency of production of knocking increases according to the advance of the engine ignition timing.
By taking this feature into consideration, it is known to provide a device to retard or to advance the ignition timing in response to the detected knocking condition by always detecting the knocking strength so as to improve the output and fuel consumption characteristics by maintaining an optimum knocking condition.
It is also known that a vibration pressure component in a particular frequency range relating to the pressure variation in the combustion chamber increases under such a knocking condition. The abovementioned conventional device generally uses this principle and comprises a sensor for detecting the vibration of an engine body. By using the detected electric signal of the sensor, the abovementioned particular frequency component or range is selected and the knocking condition is judged by obtaining a signal indicating the knocking strength. This signal is produced by subtracting a component corresponding to background noise level from the amplitude of said selected signal at that time.
More precisely, at first the aforementioned particular frequency range is separated from the detecting signal of the sensor by using a band-pass filter, then this separated signal is rectified. Thereafter the rectified signal is averaged and this averaged signal is amplified up to a predetermined level corresponding to a noise level of the background signal. Then the level of this signal obtained from the rectification is compared with that of the background signal and a pulse signal, the integrated value of which represents the strength of the knocking, is derived. By an integration of the pulse signal, the strength of the knocking or the occurrence of the same is judged.
In such a conventional system, the knocking strength is judged after various steps as have been mentioned above so it has the disadvantage that the device is complicated.